the last metal band I was in had that problem. One guitarist was using two amps. One of which was a 150 watt tube bass head!
After I got them to narrow it down to one 50 watt head each and we pulled their bass knobs back to at least 5 or 6, I could actually hear notes and play in tune!
But yeah, get a better amp. You may end up being able to cut through with treble using a 50 watt amp, but you will likely end up cooking the speaker to death in the process.
The first thing I always check is my guitar player's bass and mid knobs. Ordinarily he only touches the gain and volume knobs because he knows he doesn't know **** about EQing for a mix and he knows I do. Sometimes though he obviously has been playing by himself and I have to turn them down again.